Ai Weiwei in his Beijing studio. The artist says he has been ordered to pay £1.5m in tax. Photograph: Dan Chung for the Guardian

Ai Weiwei, the internationally renowned Chinese artist detained by authorities for almost three months this spring, has said officials have ordered him to pay 15m yuan (£1.5m) in tax.

Beijing police released him in June, citing his ill health, good attitude in confessing his crimes and willingness to repay back taxes he had evaded.

But the 54-year-old told the Guardian he did not owe the money and would challenge the demand, adding: "It's a game they play all the time … They needed an excuse to take me away for 81 days."

Ai's family and supporters always believed his detention was in retaliation for his social and political activism. During his interrogations, Ai was questioned about issues such as subversion of state power rather than his accounts.

Ai said the authorities told him the tax was owed by Fake, the company that handled his affairs but was legally registered by his wife.

"I am a designer for the company; I am not the legal representative, nor the manager," he said. "I never signed papers or made a deal or transferred money – I was nothing to do with the accounting. They said I was the 'actual controlling person' of the company."

He is unsure how he can challenge the payment notice because police seized all the company's documents a few days after detaining him and have not returned them nor allowed anyone to inspect them.

He said authorities originally told him he would have to pay 20m yuan, but later they reduced it to 15m yuan, "because they had considered my ability to pay".

They told him not to discuss it in case the figure changed, but have given him written notice that he must pay the 15m yuan and have said they will explain to him how they arrived at the sum.

Ai said public security officials told him on Monday he should not argue about the bill because the government would not change its mind. He said his mother had told him they should auction his late father's house if necessary. Ai Qing is one of China's best-known poets.

Pu Zhiqiang, Ai's lawyer, said they were seeking to challenge the demand.

According to state news agency Xinhua, Chinese police claimed during Ai's detention that Fake had evaded "a huge amount" of taxes and intentionally destroyed accounting documents.

They detained the company's accountant, Hu Mingfen, for two months. Colleagues described her as meticulous, with one telling the Guardian: "She is very prudent. Lu Qing [Ai's wife] used to say: 'Even if it's just one cent missing, Hu has to count it to be clear'."